1. Field of the Invention
The invention relates to a two-component developer used in electrophotographic copiers and printers, and more particularly to a two-component developer with high visibility and excellent durability.
2. Description of the Related Art
In the field of xerography, a conventional two-component developer is prepared by mixing a toner with a carrier. Because of the application of the carrier, the toner is hence charged to develop images. Such method has been widely adopted at present.
A carrier is charged by friction with a toner, which also makes the toner charged. In other words, the carrier surface must be designed to be capable of generating charge with opposite polarity to the toner. Magnetic materials, as a core of carriers, can be either used as the carrier themselves for independent application or coated with a resin to carry charge. In the former case, although the carrier can be simply adjusted, it is harder than the toner made by resin since the carrier surface is made of magnetic materials. When carrier surface is attached with a toner, it may cause reduction of electrification. In addition, as the carrier surface state is restricted by magnetic materials, it would be difficult to control the electrification. Other problem such as humidity dependence would arise due to conductibility of the carrier surface. To solve the problems of solidness, magnetism, and conductibility when magnetic materials are used alone, resin-coated carriers have been widely applied recently.
For magnetic materials coating, different types of resins can be applied. However, in order to achieve excellent durability of the carrier, generally resins that are wearable and difficult to be attached by toner are much preferred, specifically silicone resin, which is characterized in that it has low critical surface tension and is uneasy to be attached by toner particles. Furthermore, as silicon resin is liable to form a crosslink structure, the surface thereof will become hardened and is uneasy to be worn.
On the other hand, in recent years, many proposals have been brought forward to develop a toner by polymerization methods. As particle diameter and shape of emulsion polymerization toner, made by resin particles agglutination and fusion-spheroidization, are easy to be adjusted, such toner has been gradually used in practice.
At present, resin/coloring particles agglomeration in an aqueous medium and fusion-spheroidization is the major method to make emulsion polymerization toner, which has a uniform particle surface and electrification. It is widely popular in terms of the electrification.
However, because of high uniform surface of emulsion polymerization toner, when it is combined with the silicone resin coating to make two-component developer, its triboelectric charging performance is worse and electrified capacity cannot be too high.
When silicone resin coating carrier is combined with toner made by pulverization method, there is no problem for triboelectric charging performance and electrified capacity. Such toner is obtained by using two-axis extruder to melt, mix, pulverize and classify resin and toner. As toner surface is formed by broken-out sections, its surface would be in a non-uniformity state, under which charge formed by friction with carrier will be polarized in non-uniformity part and easily electrified. However, if we use such toner, as crushing powder and ultrafine powder exist in pulverized toner, it is impossible to prevent carrier from being contaminated and lead to poor durability of electrification effect.
On the other hand, as silicone resin is composed of hydroxymethyl silicone resin, the resin is difficult to be separated due to its polar structure. Consequently, polarization effect is less during the friction between the silicone resin and surface with high uniformity. It would take a long time for triboelectric charging because of small electrification part.